Thermionic valve circuit



May 13 1941 A D. BLUMLEIN ETAL THERMIONI C VALVE CIRCUIT Filed Nov. 7,1938 INVENTORS' ALAN D. BLUMLE/N E.L.C- WHITE BY Mg;

7.4 TTORNEY Patented May 13, 1941 UNITED STATES Parent OFFICETHERE/[IONIC VALVE CIRCUIT Great Britain Application November 7, 1938,Serial No. 239,373 In Great Britain November 6, 1937 3 Claims.

(Granted under the provisions of sec. 14, act of March 2, 1927; 357 0.G. 5)

The present invention relates to thermionic valve circuits of the typedesigned to handle signals of high frequency i. e., at such frequenciesthat the inter-electrode capacities of a thermionic valve employed inthe circuit are effective to modify the functioning of the circuit.

In a number of such circuits it is necessary to connect an impedancebetween the cathode of a thermionic valve and the negative terminal ofthe source of anode current for the valve. For example, with theso-called cathodefollower valve it is necessary to connect an impedancebetween the cathode and the negative terminal of the source and to applybetween the control electrode of the Valve and the negative terminal sothat the potential of the cathode of the valve follows substantially thepotential of the grid of the valve, the output in this case beingusually taken from the cathode impedance. Furthermore, it is usual inorder to reduce distortion in thermionic amplifiers and the like toinsert an impedance in the cathode circuit in order to provide negativefeedback for the purpose of straightening the valve characteristic andthus reducing distortion, the output in this case being usually takenfrom the anode circuit. In a particular case when a valve of the tetrodetype, which may contain electrostatic shielding plates, or of thepentode type, which has a suppressor grid, is employed in such circuitsit i usual to connect the shieldingplates, or the suppressor grid,directly to the cathode in order that the bias on the said plates orsuppressor grid is equal to the potential of the cathode. It has beenfound that when such circuits are operating at high frequencies theeffect of the additional capacity of the shielding plates, or thesuppressor grid, to ground is detrimental to the satisfactory operationof the circuits. For example, in the case of the cathode follower valvethe extra capacity added between cathode and ground will tend to stopthe potential of the cathode following that of. the-grid at highfrequencies, Whilst in the case of the negative feedback circuit alreadymentioned the feedback is reduced at high frequencies and thus thecircuits are not satisfactory.

Similar difficulties arise in thermionic valve circuits in whichimpedances are connected in both anode and cathode circuits and outputsare derived from each impedance.

It is the object of the present invention to provide improved meanswhereby these and other disadvantages are substantially overcome orreduced.

According to the invention a thermionic valve circuit of the typereferred to is provided, wherein an impedance is connected in thecathode circuit in such a manner that in operation the potential of thecathode varies, means being provided for biassing the suppressor grid,or shielding plates, at substantially the mean or low frequencypotential of the cathode While maintaining it at a substantially fixedpotential relative to ground for high frequency signals.

In order that the said invention may be clearly understood and readilycarried into effect the same will now be more fully described withreference to the accompanying drawing in which:

Figure 1 represents an embodiment of the invention as applied toacathode follower circuit.

Figure 2 represents a further embodiment of the invention as applied toa cathode follower type of circuit, and

Figure 3 represents .a further embodiment of the invention as applied toa negative feedback amplifier.

Referring to Figure 1, the reference numerals 8 and 9 represent theinput terminals to a pentcde thermionic valve l3 which is employed as acathode follower valve. The cathode I4 of the valve It is connected toasource of negative potential bymeans of resistances II and 12 which arearranged in series. The junction of the two resistances is joined to thegrid l5 of the valve by means of a. leak resistance It, the resistanceii serving as a bias resistor in order to give the grid the correct biaspotential. The screening grid N5 of the valve is taken to a suitablepositive potential by means of a resistance 1 and is bypassed to groundby condenser 6 and the anode i3 is suitably decoupled from a source ofpositive potential by means of the anode decoupling resistance l9 andthe decoupling condenser 20. The output of the valve is taken from line24, which is connected to the cathode 14.

According to normal practice the suppressor grid I 5 of the valve i3would be connected directly to the cathode M-of the valve. This may bedesirable from the operating valve conditions, since even if the anodedoes not supply a load, it enables the amount of screen current to bemore definite than is the case were the suppressor joined to the screen.In such a case itwill be seen that the capacity of the suppressor gridto ground is added to the capacity of the cathode It to ground and thusthe effective impedance between cathode and the negative source ofsupply, that is, the impedance due to the resistance i i and I2 isreduced at high frequencies and thus the potential of the cathode l4does not follow the potential of the grid l5 so efilciently as at lowfrequencies. In order to avoid this additional capacity due to thesuppressor grid, the suppressor grid I1 is, according to one embodimentof the invention, connected to a point on a potential divider 2| and 22in such a way a to give the suppressor grid I! a potential equivalent tothat of the mean potential of the cathode l4. Should the resistance 22be of a high value it may be necessary to bypass this resistance bymeans of a condenser 23. It will be seen that the suppressor gridcapacity i now removed from the cathode circuit and does notaffect theoperation of the circuit whilst the suppressor grid is maintained at thepotential necessary for it to perform its suppressor action.

A further method of avoiding the capacity effeet will now be describedwith reference to Figure 2 of the accompanying drawing. In this figurelike components to those in Figure 1 have been given like numeral and itis assumed that except for the suppressor grid circuit that the circuitbehaves in a similar manner. In this case the suppressor grid I1 isconnected to the cathode M by means of a high resistance 26, and inaddition, the suppressor grid is bypassed to ground by means of a largecondenser 25. Due to the direct current connection of the suppressorgrid I! to the cathode M by means of the resistance 26, the suppressorgrid I1 is maintained at the mean potential of the cathode l4, thecondenser 25, serving to prevent the potential of the suppressor gridfluctuating at high frequencies. Alternatively the resistance may bereplaced by or in combination with an inductance .of suitable value.

Figure 3 illustrates an embodiment of the invention as applied to anamplifier employing negative feedback in the cathode circuit. In thiscase the valve 13 has an anode resistance 28 across which outputpotentials are derived and passed out via lead 21. The resistance llserves a dual purpose of providing negative feedback for the purpose ofreducing distortion in the amplifier and for providing a suitablenegative grid bias for the grid of the valve I5. The normal connectionof the suppressor grid directly to the cathode M has the effect at highfrequencies of reducing the effective cathode impedance which determinesthe amount of negative feedback, and thus the feedbackdecreases at highfrequencies and for theseqfrequencies the output from the amplifier isincreased with consequent distortion.

.For the purpose of eliminating this effect, the

suppressor grid is connected to the cathode M by means of a resistance26 and is also connected to ground by means of a large condenser 25. Asin the case of Figure 2, suppressor grid is biassed at mean cathodepotential and the effect of the suppressor grid to earth capacity isrendered ineffective on the feedback circuit.

Alternatively, the bias for the suppressor grid I] ofFigure 3 could beobtained, according to the invention, ina similar manner to thatindicated with reference to Figure 1, namely, by connecting thesuppressor grid to a tapping from a potential divider so as to providethe suppressor grid with a bias potential equivalent to the meanpotential of the cathode of the valve.

In the negative feedback amplifier case not only is there the effect ofthe direct suppressor grid to ground capacity, but there is also theeffect .of the suppressor grid to anode capacity, thus --when the gridof the valve is positive, the potential of the anode is made negativeand thus the anode to suppressor grid capacity tends to stop the cathodefeedback circuit operating correctly at high frequencies in a similarmanner to the well known Miller effect. By adopting the circuitsaccording to the invention this effect is reduced.

A further circuit to which the present invention is applicable is thatin which a thermionic valve circuit is provided giving outputs from boththe cathode and anode circuits for example for obtaining a push-pulloutput. This case is similar to that described with reference to Figure3, only differing in that the cathode impedance is normally larger thanthat necessary for the correction of distortion. Similar ill effects dueto the suppressor grid to ground capacity occur, and this causesdistortion in that added capacity lowers the high frequency output fromthe cathode circuit and decreases the high frequency output from theanode circuit. By adopting the methods already outlined it is possibleto overcome or reduce these ill eifects.

An example of a further case in which both cathode and anode impedanceare employed is in television apparatus. In one particular application atelevision signal is applied to the grid of such a valve circuit,signals from the cathode impedance being fed to the anode of a furtherscreen grid valve used for D. C. re-insertion whilst signals in aninverted sense are passed from the anode circuit to the control grid ofthe further screen grid valve in an inverted sense for the purpose ofswitching on the screen grid valve during black level portions of thetelevision signal which succeed the synchronising pulses, the signals inthe anode circuit having passed through a delay network.

Since, in the embodiment of the invention described, the suppressor gridis connected effectively for high frequencies to the end of the cathodeimpedance remote from the cathode of the valve a certain amount ofnegative feedback will be applied to the suppressor grid of the valveand the gain will be reduced slightly from that obtainable with thenormal arrangement. In this connection, the arrangement of Figure 1 ispre-, ferred in that the feedback to the suppressor grid is constant forall frequencies handled, whereas with the circuit of Figure 2 thefeedback decreases for low frequencies.

Although the invention has been described as applied to pentode valvesit will be understood that the invention can also be applied to tetrodevalves having shielding plates (see British patent specification No.423,932), the shielding plates being connected as described above.

We claim: 1

1. A high frequency amplifier comprising an electronic tube including acathode, a control electrode, a screen electrode, a suppressorelectrode, and an anode, a source of potential having positive andnegative terminals, a resistance for connecting the cathode to thenegative terminal of the source of potential, a grid resistance forconnecting the control electrode to a point along said first namedresistance, a load impedance for connecting the anode to the positiveterminal of said source of potential, means for maintaining said screenelectrode positive with respect to said cathode, a potentiometerconnected across said source'of potential, means for connecting saidsuppressor electrode to a point along said potentiometer such that thepotential of the suppressor electrode may be equal to the averagepotential of the cathode, means for impressing an input signal upon thecontrol electrode, and an output circuit whereby input signals of highfrequency will be amplified by the tube in an amount substantially equalto the amplification of low frequency input signals.

2. An amplifier circuit comprising an electron tube which includes acathode, a control electrode, a screen electrode, a suppressor electrodeand an anode, a source of potential having positive and negativeterminals, means including a resistance for connecting the cathode tothe negative terminal of the source of potential, a grid resistance forconnecting the control electrode to a point along said first namedresistance, means for connecting the screen electrode and the anode tothe positive terminal of said source of potential, a high impedancemember for connecting the suppressor electrode to said cathode, electronstorage means connected between the suppressor electrode and thenegative terminal of the source of potential, means for impressingsignal variations upon the control electrode and an output circuit,whereby high and low frequency signal variations may be amplified by thecircuit in substantially like amounts.

3. A device for amplifying a wide band of frequencies ranging fromsubstantially zero to frequencies of the order of one megacyclecomprising an electronic tube including a cathode, a control electrode,a screen electrode, a suppressor electrode and an anode, a source ofpotential having positive and negative terminals, means including a loadimpedance for connecting the anode to the positive terminal of thesource of potential, resistance means for connecting the cathode to thenegative terminal, a resistance for connecting the control electrode toa point along said resistance means, a condenser connected between thesuppressor electrode and a point of fixed potential, a potentiometerconnected across the source of potential, means for connecting saidsuppressor electrode to a point along said potentiometer so that theaverage potential of the suppressor electrode is substantially equal tothe average potential of the oathode, means to maintain the screenelectrode positive with respect to the cathode, and an output circuit,whereby substantially linear amplification of the entire appliedfrequency range will result.

ALAN DOWER BLUMLEIN. ERIC LAWRENCE CASLING WHITE.

